Gaza’s Interior Ministry said an initial investigation revealed that large amounts of gasoline had been stored at the site, fueling the blaze that quickly engulfed the building.

Witnesses said they could hear screaming but they could not help those inside because of the intensity of the fire.

Palestinian President Mahmoud Abbas called it a national tragedy and said there would be a day of mourning.

Hussein Al-Sheikh, secretary-general of the executive committee of the Palestine Liberation Organization (PLO), said in a statement that the Palestinian Authority urged Israel to open the Erez crossing with Gaza to transport serious cases in order to treat them outside the enclave if necessary.

“The President gave instructions to provide all forms of medical and other assistance urgently,” Sheikh said on Twitter.

Tor Wennesland, the United Nations’ Middle East peace envoy expressed “heartfelt condolences” to the families of those who died in the incident, in a post on Twitter.

Jabalia is one of eight refugee camps in Gaza, home to 2.3 million people and one of the world’s most densely populated areas.